The present invention relates to a liquid-level gauge for measuring a height of liquid level and more particularly, to a liquid-level gauge for measuring a liquid-level height by generating strain in an optical fiber by the action of vertically upward force a body dipped in liquid receives from the liquid (buoyancy) and measuring the strain.
Liquid-level gauges for measurement of a height of liquid level based on various principles have hitherto been proposed. For example, an electrostatic capacitance type liquid-level gauge (JP-A-2000-097750 or JP-A-11-030544), a barometric liquid-level gauge (JP-A-2000-088629), a float type liquid-level gauge (JP-A-10-148565 or JP-A-11-326015), an electrode type liquid-level gauge (JP-A-11-023346) and an electric wave type liquid-level gauge (JP-A-10-197617) have been known. Specifically, the present invention provides a float type or comparable type liquid-level gauge, especially, using an optical fiber.
The float type liquid-level gauge detects a height of a float that ascends/descends as the liquid-level changes and it is classified into two kinds of which one uses a magnet and reed switches and the other uses a wire or a tape. The former float type liquid-level gauge has a number of reed switches arrayed in the liquid-level height direction and they are operated by the magnet as the float ascends or descends to measure a height at which a reed switch is turned on, as a liquid-level height. On the other hand, in the latter float type liquid-level gauge, a measuring wire attached to a float is wound up on a drum to calculate a liquid-level height from a windup amount.
In the conventional liquid-level gauge, especially, the float type liquid-level gauge using a magnet and a plurality of reed switches, however, it is necessary that the magnet be built in the float and a great number of reed switches be incorporated in guide pipes for guiding the float, raising a problem that the number of parts increases and the structure is complicated.
On the other hand, in the float type liquid-level gauge using a wire, many parts such as a windup drum for the wire, a windup motor and a pulley are needed, so that the gauge is increased in scale and is often troubled because of mechanical windup, thus requiring laborious and time-consuming work for repairs and maintenance.
Under the circumstances, the present inventors have studied and conducted experiments in various ways by noticing a change in buoyancy which a body receives from liquid as the liquid-level height changes in the float type or comparable type (suspension type) liquid-level gauge to confirm that the liquid-level height can be measured by detecting the change in buoyancy as a change in strain in an optical fiber.
The present invention has been made in the light of the conventional problems and the results of experiments and it is an object of the invention to provide a liquid-level gauge which can measure a liquid-level height accurately by detecting a change in buoyancy acting on a columnar body dipped vertically in liquid that is caused by a change in liquid-level height, as a change in strain generated in an optical fiber.
To accomplish the above object, a liquid-level gauge according to a first invention comprises a float having a uniform cross-sectional form in the height direction and dipped in liquid, an optical fiber having a support portion of a predetermined length that is applied with tension by supporting the float in such a manner that its upper end always projects from the liquid level, an optical fiber strain gauge for detecting a strain level generated in the support portion of the optical fiber in accordance with a change in liquid-level height, and fiber support members provided at a position engaging the top of the float and a position below the float, respectively, wherein the support portion of the optical fiber is turned round between the fiber support members.
In a liquid-level gauge according to a second invention depending on the first invention, the support portion of the optical fiber is turned round plural times between the fiber support members.
A liquid-level gauge according to a third invention comprises a float having a uniform cross-sectional form in the height direction and dipped in liquid, an optical fiber having a support portion of a predetermined length that is applied with tension by supporting the float in such a manner that its upper end always projects from the liquid level, an optical fiber strain gauge for detecting strain generated in the support portion of the optical fiber in accordance with a change in liquid-level height, and fiber support members provided at a position engaging the top of the float and a position below the float, respectively, wherein the support portion of the optical fiber is formed into a loop form between the fiber support members and fixed at its opposite ends by means of a lock member.
In a liquid-level gauge according to a fourth invention depending on the third invention, the optical fiber support portion is turned round plural times between the fiber support members so as to be formed into a loop form.
A liquid-level gauge according to a fifth invention comprises a dipping member having a uniform cross-sectional form in the height direction and dipped in liquid, a wire for supporting or suspending the dipping member in such a manner that its upper end always projects from the liquid level, an optical fiber having a portion of a predetermined length that is applied with tension generated in the wire, and an optical fiber strain gauge for detecting strain generated in the portion of optical fiber applied with the tension, wherein the portion of the optical fiber applied with the tension is laid along the wire and its opposite ends are fixed to the wire.
In a liquid-level gauge according to a sixth invention depending on the fifth invention, a portion of the wire across which the optical fiber is fixed is slackened.
In a liquid-level gauge according to a seventh invention depending on the fifth or sixth invention, the dipping member is a float having a specific weight value smaller than that of the liquid.
In a liquid-level gauge according to an eighth invention depending on the fifth or sixth invention, the dipping member is a suspension member having a specific weight value larger than that of the liquid.
In the first to eighth inventions, when the liquid-level height changes, the magnitude of buoyancy either the float or the suspension member receives from liquid also changes. Since the cross-sectional area of the float or suspension member is uniform in the longitudinal direction, the value of a change in liquid-level height is accurately proportional to a change in the buoyancy acting on the float or suspension member. Also, a change in strain in the optical fiber is accurately proportional to the change in the buoyancy. In the case of the float, as the liquid-level height rises to increase the buoyancy acting on the float, the optical fiber or wire supporting the float is tensed. Accordingly, tension applied to the optical fiber or wire increases in proportion to the buoyancy to increase strain generated in the optical fiber. In the case of the suspension member, as the liquid-level height rises to increase the buoyancy acting on the suspension member, tension applied to the wire suspending the suspension member decreases in inverse proportion to an increase in the buoyancy to thereby decrease strain generated in the optical fiber. Accordingly, by detecting strain generated in the optical fiber in accordance with a change in buoyancy by means of the optical fiber strain gauge, a liquid-level height can be measured accurately from the correlation between the liquid-level height and the buoyancy.
In the first to fourth inventions, the support portion of optical fiber is turned round or looped so that its apparent length may be shortened to permit the gauge to be reduced in size. More particularly, the support portion of optical fiber that receives tension to generate strain therein during strain measurement based on the optical fiber and for detection of the strain, the support portion is required to have a length of at least about 1.5 m. If the support portion is dipped vertically in liquid, the height of the gauge is increased and increase the size of the gauge. Thus, by turning round or looping the support portion, the vertical apparent length of the support portion can be approximately halved and the gauge can be reduced in size. With the support portion turned round or looped, the float can be supported by two optical fibers and so applied load per one fiber can be reduced. Further, by turning round the support portion plural times, the apparent length of the support portion can be shortened by the number of turns to thereby further reduce the size of the gauge.
In the fifth to eighth invention, the optical fiber can be arranged such that it is spaced apart from the dipping member. Accordingly, the optical fiber need not be dipped in the liquid with the dipping member but can be laid at a location of good environmental condition such as atmosphere, bringing about an advantage that erosion, deterioration and contamination of the optical fiber due to the liquid can be prevented.
In the sixth invention, the wire is partly slackened to prevent the optical fiber from being applied with excessive tension.
The present invention is not limited to measurement of the liquid-level height of water but can be used for measuring the liquid-level height of various kinds of liquid such as oil and medicine.